Numerous proposals have been made for systems which will enable an oil tanker to be moored and loaded at an off-shore location in order to transport oil from off-shore wells. The systems at present in use for this purpose are not satisfactory because they are very much subject to the vagaries of wind and weather and in storm conditions are unable to operate. Moreover, the rate at which oil can be loaded requires the tanker to be moored for undesirably long periods having regard to the costs of operation of large tankers.
These deficiencies of existing systems arise primarily from the fact that they use flexible pipes of limited diameter through which the oil is pumped to the tanker. The flexibility is required because of the movement of the vessel at its moorings. By the use of a new mooring system the present invention enables the oil to be conveyed to the vessel through a rigid pipe of large diameter giving a very much higher flow rate.
In accordance with the invention there is provided a method of mooring a vessel wherein the vessel picks up and hauls in a mooring hawser which is attached to a mooring post which is vertically slidable in a support located below the surface of the sea, and the mooring post is lifted by the mooring hawser into engagement with a reception chamber formed in the hull of the vessel.
For carrying out this method the invention provides mooring apparatus comprising a support unit and a mooring post vertically slidable in the support unit and having a mooring hawser attached to its upper end, the hawser having means whereby it can be located by a vessel to be moored, which can then hawl in the hawser and lift the head of the mooring post into a reception chamber in the vessel.
For use in deep water the lower end of the support unit is adapted to rest on the sea-bed in such a manner that the support unit is able to swing in any direction about the lower end, the upper end of the support unit having means for producing restoring forces tending to maintain the support unit in a substantially vertical position. The locating means may comprises a guide line attached to the free end of the mooring hawser and a float connected to the guide line.
In use the lower end of the support rests directly in the sea bed or is pivotally mounted by means of a universal pivot and the upper end lies below the surface of the sea. Only the float is present at or a little below the surface and this constitutes no hazard to shipping. The vessel to be moored can locate the float and draw the mooring post into the reception chamber and in so doing pulls itself into position over the support unit. The mooring post is attached to and supported by the vessel and can rise and fall with the vessel. The coupling of the mooring post to the vessel is, however, such that the vessel can pitch or roll relative to the post and can also swing about the post. The coupling, which can be maintained even in rough weather by reason of this flexibility is primarily by way of the hawser but in addition a resilient, preferably inflatable annular seal may engage around the mooring post. The swinging movement of the support unit and with it, the mooring post, about the lower end of the support unit, facilitates these movements of the vessel.
The invention also provides a vessel which has mooring equipment for cooperation with the mooring apparatus when the latter is in position under the sea and this mooring equipment on the vessel comprises a reception chamber within the hull of the vessel, an opening in the bottom of the vessel for the entry of the mooring post into the reception chamber, and means for hauling in the mooring hawser through the entry opening and the reception chamber to draw the mooring post into the reception chamber, a resilient gripping ring being fitted around the entry opening to engage with the mooring post.
The mooring apparatus may be constructed to enable a variety of operations to be carried out after the vessel has been moored. For example, a drill string could be run down the mooring post and the support unit to carry out drilling operations on the sea bed. In the preferred form of the apparatus the support unit is constructed as an oil reservoir for receiving oil from an off-shore well and the mooring post serves as a transfer pipe for the passage of oil from the reservoir into the moored vessel. The oil transfer can take place without pumps under the pressure of the surrounding sea water and is carried out through a rigid pipe which constitutes the mooring post and can have a large flow cross-section to produce rapid oil transfer.